This peculiar volcanic relief is the result of an effusive magmatic activity, which have occurred on site as a dome of stagnation, synonym of the more common international term “lava dome”. More specifically it is a plug-dome oriented NE-SW and made up of very compact andesitic dark grey lava, porphyritic and characterized by crystals of plagioclase visible to the naked eye and by amphiboles belonging to the hornblende group. The volcanic activity of this lava structure, as well as the other ones in the southern edge of Piana del Cixerri and around Siliqua and Vallermosa, is a demonstration of the grate magmatic Cycle which affected Sardinia from the Late Eocene until the Middle Miocene, typical of collisional geodynamic environments as those generated by the opening up of the western Mediterranean Sea and by the drift of the Sardinian and Corsican continental shield. This geological monument, dating back 27-28 million years ago, due to its almost unique morphological aspect, has been historically known for preserving on its top the ruins of the Castle of Acquafredda, built around 1274-1275 by the Pisan family Donoratico della Gherardesca, lords of Iglesias and the Sigerro Valley where, according to the legend, Count Ugolino was imprisoned.